1. Field of the Invention
The present invention relates to a camera system having automatic focusing function and an interchangeable lens structure therefor.
2. Description of the Prior Art
There are already proposed various automatic focusing cameras, one type employing a focusing system capable of maintaining even a moving object in focus by detecting whether the photographic lens is in focus, front-focused or back-focused and continuously moving the photographic lens through a lens drive mechanism in response to the detection system. The lens is called in focus, front-focused or back-focused respectively when it is focused to a principal object, or to a point closer or farther than said object to the camera. In such system, the lens drive mechanism, in response to a front-focus signal indicating that the lens is front-focused, shifts the lens toward an infinity position, which is an end position in the lens moving range and corresponds to a lens position focused to an object at infinite distance, or, in response to a back-focus signal indicating that the lens is back-focused, said lens drive mechanism shifts the lens in the opposite direction toward a shortest focus position, which is the other end position in said lens moving range and corresponds to a lens position focused to an object at a shortest focusable distance. However, if the front-focus signal continues even after the arrival of the photographic lens at said infinity position for example due to a failure in the focus detecting device, the lens drive mechanism tries to drive the lens further in response to said front-focus signal, thus eventually causing failure or damage in the lens drive mechanism or in the photographic lens itself. A similar situation may be encountered when the lens arrives at the shortest focus position in response to a back-focus signal. It is therefore desirable, in case the front-focus signal continues after the arrival of the lens at the infinity position or the back-focus signal continues after the arrival of the lens at the shortest focus position, to generate an alarm, or to forcedly terminate the drive for the lens, or to drive the lens in the opposite direction. For this purpose it becomes necessary to detect that the photographic lens is at the infinity position or at the shortest focus position and to transmit the corresponding detection signal from the lens structure to the camera.
Also in an interchangeable lens structure, it is generally desirable to minimize the number of terminals for signal transmission between the lens structure and the camera.